Mining activities in the western United States have created thousands of Acid Mine Drainage (AMD) generation sites. The contamination level of each site differs as to the number of metal contaminants and the acid content of the waste stream.
These contaminants are usually a mixture of heavy and light metals such as iron. copper zinc, magnesium, manganese, aluminum, cadmium, nickel and lead. The acid and sulfate levels vary greatly.
U.S. Pat. No. 5,698,107 to Wurzburger et al discloses a process that is capable of treating heavily contaminated acid mine water. The high metal ion content and acid levels of such waste streams are highly electrically conductive and therefore easily treated by this process.
Many AMD sites have light to moderate metal and acid loading and the AMD is not nearly as electrically conductive as the heavily contaminated waste streams.
The electrodes in the pretreatment reactors would have to be moved very close to each other in order to obtain sufficiently large current density necessary for the ion state modification disclosed in U.S. Pat. No. 5,698,107 to occur. The electrodes in close proximity to one another would greatly reduce the flow rate of these reactors and greatly reduce the treatment capacity of these systems.
There is also the problem of surface water intrusion into AMD waste streams during the winter months that reduces the conductivity of the waste stream by as much as 75% and increases the flow rate sometimes as much as 400%. Conventional AMD technologies use lime for pH control (neutralization) and provide calcium so as to precipitate the sulfates as a CaSO4 salt. These processes have common weaknesses. They contaminate tons of CaSO4 with a few pounds of hazardous waste metals making all the sludge a hazardous waste. Because the percentage of metal is so small, it is not economically feasible to try to recover the metals from these sludges.
The mine sites that are not amenable to hazardous waste storage have the added expense of placing the sludge in proper containers for hauling to a registered site, as well as requiring fees paid for sludge handling and storage. Sludge handling and storage costs are actually 300-400% of the actual water treatment costs. The use of complex thickeners to create a high density sludge can reduce sludge volumes but are capitol intensive and expensive to operate.
Previous methods of treating mine water all have limitations on their effectiveness or create such large amounts of metal contaminated sludge. These liming systems can not meet the new more stringent standards for water discharge.
U.S. Pat. No. 3,823,081 shows the use of electrolytic cells to create free hydroxide to precipitate the metals. These metals have to be oxidized with ozone or in ponds before treatment. Both treatments are very expensive and do not address removal of the remaining sulfate ions (SO4)−2 that are just a hazardous as the metal ions to plant and animal life.
U.S. Pat. No. 5,427,691 is a greatly improved version of a conventional lime treatment system and does address a means for oxidizing some of the iron and other metals. The process has the same limitations as all the other lime treatments in their inability to remove low valence metals (Cu+) or the very high valence metals such as hexavalent chrome or manganese (Cr6+, Mn6+) The greatest weakness of all these lime processes is that huge amounts of calcium sulfate precipitates are created along with the metals, converting the sulfates into a hazardous waste instead of keeping them separated so as to provide economic benefits.